The basic living unit of the body is the cell, and each organ is an aggregate of many different cells held together by intercellular supporting structures. The entire body contains about 100 trillion cells.
Each type of cell is specially adapted to perform one or a few particular functions. Although the many cells of the body often differ markedly from each other, all of them have certain basic characteristics that are alike.
Although most of this fluid is inside the cells and is called intracellular fluid , about one third is in the spaces outside the cells and is called extracellular fluid .
This extracellular fluid is in constant motion throughout the body. It is rapidly transported in the circulating blood and then mixed between the blood and the tissue fluids by diffusion through the capillary walls.
In the extracellular fluid are the ions and nutrients needed by the cells for maintenance of cellular life.
The extracellular fluid contains large amounts of sodium, chloride, and bicarbonate ions, plus nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids. It also contains carbon dioxide that is being transported from the cells to the lungs to be excreted, plus other cellular products that are being transported to the kydneys for excretion.
The intracellular fluid differs significantly from the extracellular fluid; particularly, it contains large amounts of potassium, magnesium, and phosphate ions instead of the sodium and chloride ions found in the extracellular fluid.
Special mechanisms for transporting ions through the cell membranes maintain these differences .
A receptor can combine with a highly specific molecule. (Thus, the receptor for insulin hormone will combine with insulin but not with any other molecule.)
After the combination of receptor with the specific chemical molecule for which it has affinity, a receptor-ligand complex is formed - the cell is activated or inhibited (the specific chemical molecule is called ligand).
A carrier protein is an «integral protein» of the cell membrane. It has affinity for a specific chemical molecule. For example, glucose transport across the cell occurs via carrier. There is a carrier protein in the cell membranes of many cells, which can (because it has affinity for glucose) catch a glucose molecule.
The channels, while the cell is at rest, remain closed. But these channels open when they are properly stimulated. This stimulus can be (1) electrical or (2) chemical.
Channels which open when subjected to electrical stimulus are called «voltage gated channels». Those channels which open due to action of chemicals (neurotransmitters, hormones) are called «ligand gated channels».
Some substances (eg. glucose, amino acids) are not soluble in fatty medium or cannot pass through channels because of their big size. Such substances cross the cell membrane by mediated transport (also called «carrier mediated transport»).
Some electrolytes like Na, K are transported as they can move from higher concentration to lower concentration.
At arterial end, fluid from the plasma leaves the capillary to enter the tissues. This fluid contains glucose, amino acids etc. At venous end fluid from the tissue carrying waste products enter the capillary. This movement of fluid is dependent primarily on two factors: (1) capillary blood pressure and (2) osmotic pressure of the plasma proteins.
Na, K, and Cl all can move across the membrane at rest but at different rates.
K can pass easily through its leak channels which are always open. The strong and persistent tendency of K efflux makes the inside of the membrane negative.
The tendency of Na is to enter the cell. This is firstly because the inside is negative and Na is a positive ion, secondly, the concentration of Na in ECF is many times higher than the its concentration in ICF.
But Na cannot pass through the leak channels because of its higher effective diameter along with the water molecules attached to it. The voltage-gated Na channels are also closed at rest. So, Na entry is negligible.
Movement of Cl also occurs but it is transient and insignificant.
When these forces are overwhelmed, the MP is reversed and action potential develops.
In resting state inside of the cell remains negative in respect to the outside, this is called polarised state . If this polarised state in an excitable cell like nerve or muscle is disturbed by a stimulus, then the MP changes in a specific manner due to movement of various ions leading to depolarisation of the cell .
A stimulus may be physical, thermal, chemical, mechanical or electrical in nature. The stimulus which is just sufficient to produce an AP (ie, to depolarise) an excitable cell or tissue is called a threshold stimulus.
Stimuli stronger than this threshold stimulus is called suprathreshold and stimuli weaker than this are called subthreshold stimuli.
A subthreshold stimulus will not be able to stimulate a tissue but a suprathreshold stimulus will of course stimulate.
To be stimulated a more excitable tissue needs a less strong stimulus and a less excitable tissue - a stronger stimulus.
A typical AP is seen when a nerve or a muscle fibre is stimulated by a threshold stimulus. When the stimulus is applied after some delay (latent period), there is a gradual change of MP towards “0” up to a point. This point is called firing potential (as the membrane fires at this MP).
Then a sharp change is seen and MP rapidly goes towards “0” and crosses the “0” line to become positive (+35 mV). At this point, the inside of the cell has become positive in respect to the outside. Now, the cell is said to be depolarised.
Then the MP changes in reverse direction, towards the resting value or RMP. At first it returns very quickly - called rapid repolarisation and then slowly - called negative, after potential or after-depolarisation.
After the application of the stimulus, some time is required before any appreciable change occurs in the MP This period is called latent period due to application of stimulus MP is lowered and Na channels (voltage-gated) in the membrane start opening.
Na enters the cell and reduces the MP further. This lowering of MP opens up more Na channels. It continues in a positive feed back manner (i e more Na enters more Na channels open up) upto a point and the MP reaches a critical value - firing potential .
Then there is opening of maximum number of Na channels. It leads to explosive Na entry and the rapid upshot of MP and depolarisation. So, the MP is now positive, the cell is depolarised and the cause of this depolarisation is explosive Na entry.